1. Field of the Invention
The present invention relates to the field of magneto-optical recording systems such as a magneto-optical disk drive and, more specifically, to a system compatible with different types of magnetic-optical media, particularly a write-after-erase type and a direct-over-write type.
2. Description of the Related Art
Magneto-optical disk drives are devices for recording/reproducing data to/from magneto-optical media using a laser beam and are widely used as a peripheral storage devices for computers. A magneto-optical disk cartridge can be inserted into and removed from the magneto-optical disk drive and accommodates therein a rewritable magneto-optical disk medium. Cartridges that comply with the ISO standard and store R8 or 230 megabytes (MB) in 3.5 inch magneto-optical disk cartridges are widely used. Such cartridges are only slightly thicker than conventional 3.5 inch floppy disks, but hold up to 160 times the data. Moreover, such cartridges complying with ISO standard for a 640 MB magneto-optical disk cartridge are currently being discussed. Magneto-optical disk cartridges are expected to further expand their memory capacity.
The typical magneto-optical disk media includes a single magnetic layer on the substrate. Data is stored in the disk by local magnetization of the magnetic layer. The disk drive records data using a laser beam and a magnetic field. The laser beam when set at a relatively high power for recording heats the magnetic layer up to a temperature higher than the Curie point of the material of the media, and the magnetic field can flip or change the state of a small region where the magnetic layer is heated to a higher temperature than the Curie point. The disk drive reproduces data using a laser beam and a photo sensor. The photo sensor detects the laser beam reflected from the medium and polarized by the Kerr effect of the magnetic layer. Such a medium is disclosed, for example, in the Japanese Patent Laid-open No. SHO 58-73746.
Typical magneto-optical disk drives are classified into two types based on recording method: magnetic modulation recording and optical modulation recording. In the magnetic modulation recording method the direction of the magnetic field can be changed based on the input data. Data is recorded by continually irradiating the media the laser beam at write power level (a Curie power level) and by selectively changing the direction of the magnetic field in the media responsive to data to be written. This recording method has an advantage in that the disk drive directly can overwrite the data on the disk but has a disadvantage in that the disk drive cannot perform the overwrite at high speed because it is difficult to manufacture an electromagnet which is sufficiently compact, at a reasonable cost and which allows high speed operation.
In the optical modulation recording method the disk drive changes the power level of the laser beam responsive to data to be written. Initialization data (erase data) that sets the magnetic media to a known state is first recorded by continually applying a magnetic field to the media having a first direction and continually irradiating media with a laser beam at an erase power level (a Curie power level). Then, data to be written is recorded by continually applying a magnetic field having a second direction to the media and selectively irradiating the media with the laser beam at a write power (a Curie power level) responsive to the data. The advantage of this recording method is that the disk drive can erase data or write data at high speed because it is possible to change the power level of the laser beam faster than it is possible to change the direction of the magnetic field. The disadvantage is that the disk drive uses a two process of erasing and writing. However, this disadvantage has been reduced somewhat by using a write-cache memory for this type of drive.
A still newer type of media has been recently proposed called a direct-over-write. This newer direct-over-write media of the optical modulation recording type has been recently proposed but has not yet reached the commercial market. Data is directly over-written by continually applying a magnetic field having a predetermined direction to the media and by selectively irradiating the media with a laser having one of a first write power level and a second write power level responsive to the data to be written. Such a medium is disclosed, for example, in the Japanese Patent Laid-Open Nos. SHO 63-268103 and HEI 4-192135.